Mystery Solved!
Solution: The rocks slide across the playa on a sheet of ice with the help
of aerodynamic forces, which are produced by natural wind tunnels.

Sliding Rocks on Racetrack Playa

by Dennis Brooks

Terrain

Racetrack Playa is a lake-bed, which is dry most of the time and is nearly
perfectly flat. The Playa is located between the Cottonwood Mountains to
the East and the Last Chance Range to the West. Its elevation is 3608 feet
above sea level. The floor is about 2.8 miles long (north to south) and
about 1.3 miles wide (east to west). It is nearly level with the North end
being only 1.5 inches higher than the South end.

At the northern end of the lake-bed is a rock formation known as the
Grandstand. Rocks from the Grandstand and other rock formations break off
and fall onto the lake-bed. These Rocks come into the Playa from all around
the lake giving it plenty of rocks of all shapes and sizes. Some of the
rocks are boulders that weigh up to 700 pounds. The rocks are scattered all
across the lake-bed.

The surface of the floor is covered with cracked mud, which consists of
silt and clay. As the mud dries, it shrinks and breaks into interlocking
polygon-shaped tiles. During the rainy season, enough water comes in to
fill the cracks and cover the tiles. From a distance, it looks like a huge
lake.

The water leaves deposits of mud and silt behind, which settles as a thin
layer over the floor and in between the polygon-shaped tiles. When heavy
rains come during the winter, the tiles are temporarily erased because the
mud fills in the cracks. In the spring, dry conditions cause the mud to
crack again and form new tiles where the old ones had been.

The Conditions

When it rains in the area, the mountains surrounding
Racetrack Playa produce runoffs that cover the Playa floor. During the hot
summer days, the rain water transforms the surface of the Playa into a very
soft and slippery mud. However, the water evaporates quickly because of the
heat. When the water evaporates, the mud dries, shrinks, and cracks into
the polygon-shaped tiles. When it rains at night, the shallow lake turns
into a sheet of ice because of the extreme temperatures at such a high
elevation. The thickness of the ice depends on the depth of the water at
the time it freezes. When the sun comes up the next day, first the ice
turns into water and the drying cycle starts over.

The prevailing winds in the area blow across Racetrack Playa from southwest
to northeast. During winter storms, the winds sweep down from the
surrounding mountains across the Playa at speeds up to 90 miles per hour.
This sets up a natural wind tunnel across the Playa and simulates the
conditions created by researchers who test-fly aircraft indoors.

Moving Rocks

The scattered rocks and boulders move across the floor from time to time.
They slide across the Playa, cutting trails in the sediment as they go.
Rocks, stones, and boulders of all sizes and shapes are found all over the
floor of the playa. Many of them have long trails behind them. This gives
the impression that the rocks are racing across the floor when no one is
watching. That is how the Playa got its official name, The Racetrack.
However most people simply refer to it as Racetrack Playa.

Trails

Thanks to the observations and reports visitors bring back from
their trips to the area, we can get a detailed analysis of the rocks'
movement. Visitors bring back photos taken from every angle of the rocks
and their trails. They also add their comments, which help us see what
takes place on the Racetrack. All of this help us develop a collage of the
rocks' movement. We find these images and comments on the internet and use
them in our research.

The rocks sitting at the ends of the trails tell a story of their playful
activities. The trails range in length from a few inches to over 2,500
feet. Rocks with rough bottoms leave straight tracks with multiple groves
while those with smooth bottoms may wander about over the surface as they
travel. Some of the larger boulders slide along leaving a very shallow
trail or no trail at all.

Sometimes a rock will turn over while traveling. Then it continues to move
making a new trail with its other side.

Some visitors report that they have seen some rocks that appeared to have
dropped out of the sky. These rocks leave only a crater and no trail at all.
The shape of trails behind the rocks show that the rocks move when the
floor of the Racetrack is covered with a very soft mud.

Scientists have observed that when the rocks move, there is always wind and
water and that most of the rock trails are inline with the direction of the
prevailing winds. The longest and straightest trails are more numerous in
the southeastern part of The Racetrack. This is where the wind is channeled
through the mountains to form a natural wind tunnel. Two other wind tunnels
cross the Racetrack near its center and create turbulence in the area. The
area where the two wind tunnels converge is where the rock trails are the
most convoluted.

The rocks seem to defy nature. They slide in straight lines, go in circles,
and zig zag across the floor. Even though the lake bed is considered to be
flat, the elevation at one end is slightly higher than the other. Since the
rocks are moving in all directions, sometimes they travel slightly uphill.

Explanation

It is the wind that creates the primary forces that move the rocks. The
sheet of ice that forms over The Racetrack at night provides the surface
conditions that allow the rocks to move so smoothly across the floor of the
lake bed.

When the winds comes down through the mountain pass at 90 miles per hour,
two thing happens. First The Racetrack is transformed into something that
closely resembles a tarmac rather than a dirt racetrack. When the wind
sweeps over the sheet of ice, aerodynamic conditions above the surface of
the ice are created. The conditions are similar to the conditions an
airplane experiences as it travels through the air.

These conditions can be compared to an airplane's flight because of one
rule. An airplane will fly whether it is moving through the air or whether
it remains stationary and the air is moving over its wings. Thus, the wind
tunnel. The wind tunnel creates a low pressure area on the already slick
ice. This makes the surface of the ice almost frictionless. Without much
friction to stop their movement, the rocks slide along with the force of
wind blowing them from one location to another.

Sure, the aerodynamic forces will fly the rock over the surface of the ice
after they start moving, but what gets them airborne from the beginning? It
takes a considerable amount of force to dislodge a large rock from its
resting place and set it in motion. So, how can a 700 pound rock go from a
body at rest to a body in motion? Also, some of the rocks are blocked by a
mud barrier produced by their last stop. This makes it even more difficult
to move. The force must not only get the rock moving, but it must also have
enough power to break through the mud barrier from a resting position.

The second thing that happens in the wind tunnel is that the rocks
experience the same aerodynamic forces that the ice sheet experiences.
However, the low pressure area created over the rocks gives them lift. If
the rocks have a smooth surface, the wind will lift them up onto the ice
and send them sailing down The Racetrack. If the conditions are just right,
a smooth rock may go airborne and fly across The Racetrack making a crater
when it lands.

That explains how small, smooth rocks get their start, but what about the
big jagged rocks? How do they get started? The big rocks get their start
from a force called parasitic drag. Where aerodynamic lift allows aircrafts
to fly straight, parasitic drag causes them to crash. Fortunately, on The
Racetrack, these two opposing forces work together to give the big rocks
their start.

When the wind sweeps across a smooth rock, the wind-stream separates
briefly starting at the tail of the rock; then it comes back together again
at the face of the rock. The separation of the wind-stream causes the low
pressure on top of the rock and results in its lift. With large, flat-faced
rocks, the wind-stream separates, but it does not come back together except
downstream. This condition creates parasitic drag.

Also, instead of the low pressure forming on the top of the rock, it forms
at the face of the rock. This parasitic drag is not just a little low
pressure; it is a virtual vacuum. When the wind reaches a high enough
speed, the big rock is sucked into this vacuum to start its journey. After
it gets started moving, it continues to move with the wind. Sometimes it
shaves ice along the way and pushes it ahead as it goes. The shaved ice may
build up and cause enough resistance to stop the rock's movement. At other
times, the big rocks fly across the floor with nothing in their way.

Answers

When the wind dislodges rocks, some of the rocks break through a mud
barrier and the barrier separates into four or five smaller clumps. When
the barrier breaks, the clumps of mud are already in motion. The wind
continues to sweep the clumps along with the bigger rock. Sometimes their
trails diverge slightly, and their tracks are shorter and not as deep as
some of the other rocks. Visitors have observed that the mud tracks are
generally parallel and have even spacing.

Also, smaller rocks generally make deeper tracks because they can move
along with only a thin sheet of ice between them and the soft mud. As they
move, the ice breaks up over the soft mud and leaves a deeper, wider trail.
Since heavier rocks generally need to move on thicker ice, by the time they
start moving, the ground below the ice-sheet is also frozen solid giving
them a much harder surface on which to travel.

According to the National Park Service, there are 8 other playas near Death
Valley. This phenomenon occurs at every one of them because they all have
clay-bottom lake-beds, enough moisture to make the surfaces slick, loose
rocks, and strong winds to move the rocks around.

Solution: The rocks slide across the playa on a sheet of ice with the help
of aerodynamic forces, which are produced by natural wind tunnels.

About the Author

Dennis Brooks retired from the US Army in 1990 and attended Drury
University (Springfield, Missouri) where he received his bachelors of
science degree. After graduation, he started doing independent research on
various subjects including AIDS, dyslexia, and dieting. He worked as a
teacher in public schools, and as a private tutor, teaching reading. He is
presently retired and spends most of his time writing and surfing the
Internet looking for mysteries to solve. He also studies physical science
and astronomy. He hopes to someday write a book on unsolved mysteries.

Contact the author:dennisbroo@gmail.com

References:

The Mysterious Sailing Stones of Death Valley
http://www.dunego.net/amazing/37-nature/79-the-mysterious-sailing-stones-of-death-valley.html